Digital control offers attractive features that can result in significant enhancements of low-power switch-mode power supplies (SMPS). Digital realization allows development of new control techniques that increase overall efficiency of power stage through multimode operation, enable active monitoring of SMPS parameters and subsequent auto-tuning and improve transient response by avoiding gain and parameter variation problems characteristic for analog implementations or using nonlinear control techniques. Also, with the support of automated design tools and hardware description languages (HDL), digital systems can be designed in a short time and easily modified. These tools also allow simple transfer of the designs from one implementation technology to another, i.e.: design portability. This is a highly desirable feature in modern IC design where the chip implementation technologies are changing constantly.
In spite the fact that all of these characteristics are very suitable for low-power applications, in miniature battery-powered devices such as mobile phones, PDA-s, and MP3 players, PWM analog controlled SMPS are almost exclusively used. This is mostly due to the absence of low-power digital architectures that can support operation at constant switching frequencies significantly higher than 1 MHz. The power consumption of the existing digital controllers is often comparable to that of the supplied low-power electronic loads resulting in a poor overall efficiency of the SMPS. At higher switching frequencies the analog controllers take much less power, and consequently are more suitable solution, even though they do not posses most of the abovementioned features. One of the main limitations to maximum switching frequency at which digital controllers can be effectively used in low-power applications is analog-to-digital converter (ADC). Conventional high-speed ADC architectures are usually not suitable solutions.
One problem with conventional ADCs used in low-power dc-dc converters is their poor utilization in terms of performance. Conventional devices usually operate only around one operating point, corresponding to the output voltage of the power stage, which is usually constant.